The present invention relates generally to Sallen-Key filter circuits, and is particularly concerned with tuning of such circuits.
Component variation from chip to chip is a widely recognized problem in integrated circuits (IC) design. One widely recognized technique for dealing with this problem is the tuning of components under digital control until the error due to component variations becomes negligible. In a typical RC filter composed of amplifiers, resistors and capacitors, the accuracy of filter transfer function is determined by the resistor and capacitor values. In general, either resistors or capacitors can be tuned to obtain the required overall transfer function. Tuning can be performed efficiently by switching small-valued components in or out of the circuit. As the dominant non-ideality of a reasonably-sized micro-switch is its parasitic resistance (up to very high frequencies), tuning of capacitors poses a difficulty as a result of an undesired resistance appearing in series with the capacitor to be tuned. Tuning of resistors, therefore, can be more effective in many realizations. A resistor is commonly tuned by switching in or out some small-valued resistors that are in series with the resistor to be tuned. Switches are closed and opened to include more or less resistance in series with the resistance to be tuned. When fine tuning is desired, the switched resistors need to be much smaller than the resistor to be tuned, typically on the order of one hundredth or less. This necessarily requires large size switches, such that the parasitic switch resistance can be neglected next to the tuning resistors. A typical example is a 5k resistor to be tuned to below 1% precision. This requires less than 50 ohm tuning resistors, which in turn requires a switch resistance on the order of 10 ohms or less. A switch with such a low on resistance requires a transistor that is several hundred times larger than a minimum geometry device. Note that, increased switch size, besides requiring more chip real estate, also exhibits higher parasitic capacitance along the signal path, and increased noise coupling through the substrate. The large spread of resistor values also limits the accuracy and matching between resistors. Small-valued resistors also require much more hand-tailoring in layout, as their aspect ratios turn out to be awkward, and parasitic contact resistances introduce considerable error to overall resistance. Another difficulty is that different tuning resistor values are needed for each different resistor value to be tuned (such that the same relative accuracy can be maintained across all resistors). For example, 50 ohm resistors are needed to tune a 5k resistance, whereas 75 ohm resistors would be needed to tune 7.5k resistor with the same relative increments.
A Sallen-Key filter has an operational amplifier of unity gain connected in a circuit with two resistors and two capacitors. It is normal to tune both resistors and both capacitors in such filter circuits in order to tune the overall filter response. However, such arrangements are complex and ineffecient.
It is an object of the present invention to provide a new and improved filter circuit with a built-in tuning assembly.
According to one aspect of the present invention, a filter circuit is provided which comprises an operational amplifier connected in unity gain, the amplifier having a positive input, a negative input, and an output, and at least one adjustable resistor connected in series with the amplifier input, the adjustable resistor comprising a resistor ladder network, suitably an R-2R ladder, the R-2R ladder having an input and first and second output terminals, a plurality of arms connected in series with the input and first output terminal, each arm having a resistor R and a node separating each pair of adjacent arms, a shunt arm connected to each node, each shunt arm having a 2R resistor and a switch in series, each switch having a first, closed position connecting the shunt arm to the first output terminal and a second, open position connecting the shunt arm to the second output terminal, such that the resistance of the ladder network is varied dependent on the switch positions and is at a maximum value when all switches are closed and a minimum value when all switches are open, whereby a resistance can be tuned by varying the switch positions in the ladder circuit, the first output terminal being connected to the amplifier output and the second output terminal being connected to the amplifier positive input.
The filter circuit is of the Sallen-Key type and, in an exemplary embodiment, two resistors are connected in series between the circuit input and the amplifier, with the second resistor comprising the adjustable R-2R ladder network, and a first capacitor is connected between the junction connecting the two resistors and the amplifier output. A second capacitor is connected between the second output terminal of the R-2R ladder and ground.
Although either resistor may be used for tuning the Sallen-Key filter circuit, the second resistor is more suitable as one of the terminals is at virtual ground. Due to the unity gain configuration of the amplifier in this circuit in which the output is connected to the negative input of the amplifier, the two output terminals of the R-2R network can be maintained at the same potential simply by connecting one to the amplifier input and the other to the amplifier output. This allows a Sallen-Key filter circuit to be tuned effectively by means of a single tunable resistor network.